Crosslinked double base propellant compositions generally comprise a urethane rubber binder plasticized with nitroglycerin and filled with particulate solid fuels and oxidizers. The urethane rubber binder employed in such propellants comprises a low molecular weight nitrocellulose and a low molecular weight polyester polyol crosslinked with a diisocyanate. Due to the low molecular weight and high hydroxyl functionality of the nitrocellulose and of the polyester polyol employed in prior art urethane rubber binder compositions, the potential crosslink density of such binders is much higher than is desirable for good elastomeric quality.
Attempts to avoid the undesirable consequences of over crosslinking have been based on the use of amounts of diisocyanate crosslinker considerably less than is sufficient for a stoichiometric urethane reaction. In this method of attempting to avoid over crosslinking, substantial portions of the nitrocellulose and the polyester polyol in the composition are unreacted and therefore unintegrated in the urethane binder polymer network resulting from the crosslinking reaction. Any partially unreacted nitrocellulose or polyester polyol in the binder polymer network produces a side chain mass which is detrimental to elastomeric quality. Additional side chain mass can result from partial reaction of the diisocyanate crosslinking agent with, for example, other components of the propellant composition such as stabilizing agents, which reactions often cause chain termination. Still other side chain mass is produced by cyclization of the polymer network resulting from high dilution (low concentration) of reactive species by the plasticizer.
As a result of the occurrence of such reactions, solid propellants prepared utilizing state of the art urethane rubber binder have marginal or unsatisfactory mechanical properties and fail to qualify for certain rocket motor performance specifications. Mechanical properties of propellants utilizing state of the art urethane binder systems, especially strain capability, are severely limited as the solids loading of such binder systems is increased above about 70%.
It is an object of this invention to provide an improved crosslinked binder composition for crosslinked double base propellant having improved mechanical quality, reliability and safety over the state of the art crosslinked double base propellant binder compositions.
It is another object of this invention to provide an improved urethane rubber binder composition for use in crosslinked double base propellant compositions in which effective crosslinked density control in the binder can be established, and in which solids loading greater than about 70% by weight can be achieved while retaining satisfactory mechanical properties.